This invention relates to a deflecting voltage generating circuit for applying a predetermined voltage across the deflecting electrodes of a streak tube or sampling streak tube or the like.
A deflecting voltage generating circuit for applying a predetermined voltage across the deflecting electrodes of a streak tube or sampling streak tube or the like has been well known in the art.
FIG. 4 is a schematic diagram showing the arrangement of a general sampling streak tube. The sampling streak tube 150, as shown in FIG. 4, comprises: a photocathode 131 to which fluorescene is applied; a mesh-shaped electrode 135 for accelerating an electron beam emitted from the photocathode 131; a focusing electrode 136 for focusing the electron beam thus accelerated; an aperture electrode 137; deflecting electrodes 133 for deflecting the electron beam which has been focused by the focusing electrode 136 and passed through the aperture electrode 137, in a sweep mode; a microchannel plate 132 for multiplying the electron beam thus deflected; a sampling electrode 151 for sampling the electron beam thus multiplied; and a phosphor screen 134 to which the electron beam thus sampled is applied.
When, in the sampling streak tube 150, an incident light beam having a constant repetitive frequency is applied to the photocathode 131, the photocathode 131 emits an electron beam corresponding to the intensity of the incident light beam. The electron beam thus emitted is deflected by the deflecting electrodes 133 and sampled with the slit of the sampling electrode 151, as a result of which the parts of the waveform of the incident light beam are successively observed; that is, the variation with time of the waveform of the incident light beam can be detected as a time-series spatial image on the phosphor screen 134.
The deflecting electrodes 131 of the sampling streak tube 150 are connected to a deflecting voltage generating circuit 152. The circuit 152 is provided to apply a predetermined deflecting voltage to the deflecting electrodes 133. The deflecting voltage is trapezoidal in waveform, and is substantially in synchronization with the repetitive frequency of the incident light beam; however, it is gradually shifted in phase so that the parts of the waveform of the incident light beam may be successively sampled.
The deflecting voltage outputted by the deflecting voltage generating circuit 152 must be large in amplitude so that the background noises may not be sampled which are caused by the photoelectrons attributing to light beams scattered by the mesh-shaped accelerating electrode 135 during the period that the electron beam is not deflected. Furthermore, for the purpose of obtaining a high through rate, the drive voltage should be high. In addition, since no sampling operation is carried out inside of the general streak tube, the deflecting voltage must meet not only the above-described requirement that the deflecting voltage should be large in amplitude, but also the requirement that the waveform thereof should linearly rise and fall.
In order to satisfy these requirements; that is, in order to produce a deflecting voltage large in amplitude and excellent in linearity, the conventional deflecting voltage generating circuit 152 employs a switching element such as an avalanche transistor, avalanche diode or triode.
In the case where the deflecting voltage generating circuit 152 is made up of the above-described switching element for the purpose of high speed sweep; that is, it is intended to provide a voltage waveform steep in rise, or high voltage/time ratio (through rate), it is necessary to use a high drive voltage for the switching element, because the switching speed of the switching element is of the order of several nano-seconds. The drive voltage is, for instance, of the order of several kilo-volts (KV). Therefore it is necessary to provide a driving power source for generation of a high drive voltage and its insulated wiring for the conventional deflecting voltage generating circuit 152.
On the other hand, in general, it is desirable to observe an incident light beam with an extremely high repetition frequency (for instance 4 GHz) with a high time resolution of several picoseconds. In the case where such an observation is performed with the above-described streak tube or sampling streak tube, the electron beam sampling repetition frequency must be considerably high. For example, it is necessary that the deflecting voltage generating circuit 152 outputs a defecting voltage having a high repetition frequency (for instance 4 MHz).
However, the switching element such as an avalanche transistor employed in the conventional deflecting voltage generating circuit 152 has a long restoring time after operated and a long charging time for high voltage, so that the deflecting voltage generating repetition frequency is limited, for example, is of the order of 10 KHz in maximum.
In addition, the timing between a trigger signal for starting the switching operation of the above described switching element and the deflection voltage output cannot be electrically adjusted, and therefore it is necessary to additionally provide a delay circuit therefor.